Linear Rail X Gantry Support (MGN12)
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@deckingman @gtj0 @o_lampe @mrehorstdmd
Regarding designing a coreXY belt system. When the toothed face of a belt runs around an idler pulley, should that idler also be toothed, or doesn't it make any difference?
Dizzwold.
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@Dizzwold It depends on the pulley diameter. I once had a link to a white paper from Gates that indicated that at least 9 teeth need to be in contact with a smooth pulley. In a corexy mechanism the belts are usually bend 90 degrees around the pulleys. That means you'd need 36 teeth around the circumference of the pulley. Ignoring the fact that the teeth get closer together when you bend them around the pulley, it means you need the pulley circumference to be 72 mm (36 teeth x 2 mm pitch). That means the diameter should be 72/pi= 22.9 mm. I used 608 bearings that are 22 mm in diameter and it works fine- no artifacts from the belt teeth in the prints.
608 bearings are cheap and readily available, but they are a bit heavy and require an 8mm axle that's also heavy. You can use smaller, lighter, smooth pulleys (625s, for example) if you put twists in the belts so that the smooth back sides of the belts contact the pulleys. The machine will also be quieter at high speeds if you twist the belts. The twists can go in the belt segments that run from the drive pulleys to the corner pulleys, but you have to make sure there's clearance for the ends of the x axis as it passes by the twists.
Gates has some recommendations for twisting the belts that I don't recall exactly, but it has to do with the belt width and the length of the twisted segment. Wider belts require longer segment length for the twists. If the bed in your printer is 300 mm in Y, I believe the segments between the drive and corner pulleys will be long enough for the 180 degree twist required to use pulleys in contact with the backs of the belts, even if you use 10mm wide belts.
You can buy small, toothed pulleys that have tiny bearings, but I wouldn't expect those bearings to last very long. You're better off maintenance and reliability-wise using larger bearings as the pulleys.
Make sure you use quality drive pulleys! You can get Gates pulleys for just a few $ each from Filastruder and they really are better than the cheapos from China.
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@mrehorstdmd
Hi Robert,
Sorry, My typo.
Hi Mark,Thank you for that. Rotating the belts seems like a very good way to go as I've been getting complaints from my neighbour.
The smaller bearing you recommend is that a typo and should be F625Z, as I can't find any 625S ?Dizzwold.
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Hi Mark,
Sorry for the name mix-up above.
I've been reading up on your CoreXY belt configuration documents.
Referring to your Belt Segment Diagram;
Do the belt segments C, D, E & F have to be separated, to the front and back of the X carriage, or is it possible that as long as the belts are parallel they can all be mounted in the same position, either all on the front (as demonstrated below), or all on the back of a carriage plate with using linear rails (MGN12)?
Would this work, or would there be a possibility of twisting/unwanted wear?Dizzwold.
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@Dizzwold That's how I do mine. In my case, the belts run on the centre line of the X rail. To my way of thinking, the big advantage is that the belt tension does not impart any twisting force, even if the tension is unequal (but of course it should be equal). With the belts offset front and back, when you adjust the tension on one belt, it will twist the gantry so you have to adjust the tension on the second belt to bring it back square but that then affects the tension of the first belt etc....
For info, I documented my recent CoreXY rebuild on YouTube and created a playlist which you can reach here https://www.youtube.com/playlist?list=PLMt2DqJCk4V95a5sZzkpzvL-vM4G_JMxI
You might pick up some tips or ideas that might help................
Edit. I don't twist my belts but have never had any issues with toothed side against idlers - probably because the OpenBuilds idlers I use have quite a large circumference compare to say, a pulley.
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I chose to stack the pulleys at the ends of the X axis, resulting in the belts being separated on the extruder carriage. If you prefer to keep the belts in the same plane on the extruder carriage, you can separate the pulleys as @deckingman has done. You can also used stacked pulleys at the corners as I have done, or separate them. I found it easiest to build the machine using stacked pulleys everywhere, so that's what I did. It did make the extruder carriage design a little more complicated, and probably not suitable for rapid or automatic tool changing.
When the belt teeth contact the smooth pulleys at high speeds they make a zipping noise at each pulley. It is quieter to twist the belts at the J and K segments so the smooth back sides of the belts contact the smooth pulley surfaces.
I meant F625 bearings (the "s" was to indicate plurality). I used F625 pulleys in my sand table that runs at 1000 mm/sec along the edges of the drawings and it has been running reliably for >2 years with drawing files that often move the magnet a few km before a drawing is done. F625 bearings are commonly used in motors (NEMA-17 steppers, for example), so they are readily available and cheap and come in a variety of grades. I used "motor grade" bearings in the sand table.
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Hi Guys,
When using 2 flange bearings to double as an idler (SF605ZZ in my case), would you put a shim between them?
Also my X gantry is looking like it'll be 530mm. would a 2020 profile this length be strong enough to carry 800g?
Dizzwold.
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@Dizzwold said in Linear Rail X Gantry Support (MGN12):
Hi Guys,
When using 2 flange bearings to double as an idler (SF605ZZ in my case), would you put a shim between them?
Yes, I would - definitely. You need to ensure that each bearing is free to rotate in isolation. All open builds wheels and idlers use two bearings and there is always a shim between them.
Also my X gantry is looking like it'll be 530mm. would a 2020 profile this length be strong enough to carry 800g?
I did the calcs (which are a bit tedious) for my machine. Here is a link to a resource that'll help https://openbuilds.com/threads/how-to-calculate-v-slot
-deflection.4881/For my machine, the X gantry rail is 600mm between supports. I can't off hand remember the exact mass but it is similar to your 800g. For that length with that mass, the deflection using 2020 was unacceptable. Again, I can't remember the exact numbers but I would consider anything greater than 0.15mm deflection as unacceptable, 0.1 to 0.15 borderline, and below 0.1mm OK. 4020 was OK and that's what I used (the 40 being the vertical dimension so 20mm wide, 40mm tall).
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Cheers Professor Ian,
I wasn't to sure on the shims, and with watching your new build and image above it made me wonder about the length of my proposed X Gantry (530mm and roughly 800g extruder parts), and the use of 2020 profile for this purpose.
I'll have a read of the linked thread above.Thank you for your input and advice.
The link above to different profile types and deflection is a very good find. It's a shame they don't go any further with beam types such as aluminium angle etc. But non the less very informative.
Dizzwold.
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I may have given you bum information.
But first, if you follow the link I posted it'll take you down a rabbit hole and you need to have an account and sign in. If you get through that process, it'll take you a page where you can download a spread sheet. To save you the trouble, I've downloaded it and then re-uploaded it to my Google Drive. Here is a shareable link to that spread sheet.
It seems that you need to put an "x" in the box next to the profile, then enter the "beam" length and mass. You want to look at "Simply supported bean with concentrated load". I put in length of 0.6 (metres) and mass of 0.8 (Kgs). I haven't checked the calcs but assuming the formulas are correct, then the maximum deflection is only 0.07mm for 2020 extrusion so you'll probably be OK with that.
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A piece of same-size (3/4") square aluminum tubing would likely be lighter and stiffer than a piece of 2020. T-slot is nice stuff to use when you don't know what you might want to attach to it in the future, but is about as rigid as a wet noodle. You know what you're attaching, and where, so you don't really need to trade t-slot convenience for rigidity.
The trick with tubing is to figure out how to bolt your linear guide to it without having to drill a lot of big tool access holes. Fortunately you have a 3D printer so it isn't a problem. What you do is print a plastic piece that will hold a bunch of nuts (M3?) to match the screw spacing for mounting the linear guide, and will fit inside the tube. Then drill holes one side of the tube that will just clear the screws. Now slide the plastic piece into the tube and align the nuts with the holes and mount the linear guide, then slide the plastic tool back out of the tube.
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@mrehorstdmd Ivan Miranda @Ivan-Miranda does a great job of making things out of square tube! eg https://www.youtube.com/watch?v=qnOci3cJapQ
He tends to drill and tap the tube, but I guess that's because he's using thicker walled tube.Ian
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Hi Ian,
Just to revisit this before I consider getting some items machined;
What, if any gap should I add to the hole for the extrusion to fit. I'll be using something along the lines of your solution of adding a T nut perpendicular to the extrusion, but should the hole be machined exactly to the dimensions of the extrusion?Hi Mark,
The paper you mentioned on the Gates Timing belts, was it the following I've found;
https://drive.google.com/file/d/1POritr7FPCOrrN1QoxM0UfkxK0Y2iGMk/view?pli=1 -
@Dizzwold said in Linear Rail X Gantry Support (MGN12):
Hi Ian,
Just to revisit this before I consider getting some items machined;
What, if any gap should I add to the hole for the extrusion to fit. I'll be using something along the lines of your solution of adding a T nut perpendicular to the extrusion, but should the hole be machined exactly to the dimensions of the extrusion?If you are going to clamp it in place, then a small amount of clearance will make assembly easier. I'd give your machinist a tollerance of -0, +0.5 mm. I.e the hole must be no smaller than than the extrusion but can be up to 0.5mm larger. It's important that the distance from the bottom of the block to the bottom of the hole must be the same for both blocks otherwise the rail will be higher one end than the other.
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@Dizzwold Thanks! Page 63, bottom right - smooth idlers should have diameter equivalent of the pitch diameter of a 40 tooth pulley. Page 18 shows the pitch diameter of a 40 tooth pulley (for 2mm pitch belt) is 25.46 mm. So the 608 bearings are a little small at 22 mm, but I haven't seen any issues with them.
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Hi Ian,
Thank you for your advice. I'm most grateful.
Hi Mark,
Glad to help and actually contribute for a change.
Are either of you moderators, or can put the Gates Timing Belt info in a file somewhere, where everyone can access it easily from the forum main page or something?
I think most users of the forum use Gates belts (or similar), so could be very handy for others.Thanks again,
Dizzwold.
